PHP GD If/Then Behavior

I'm pretty new to PHP in general, and at the moment I'm working with the code snippet below trying to answer what should be a simple question:

When the "inputtext" variable exists, passing the var to the GD imagestring function, the imagepng($im) outputs the combined text on the image.......but what keeps it from being accessible by "right-click/Save picture"?

I'm having difficulty figuring out the right question to ask of he documentation, and while I'm thinking that the simplest soluton will involve output buffering, I haven't been able to get an understanding of the topic enough to see how it would be applied here......

Any pointers in the right direction would be ever so much appreciated.....

Goal: Have form that asks for text, then submits that text to a GD imagestring, putting the submitted text into/on the graphic.

What's done so far: Put the form and the GD function into a single script. Started by having the GD script in a separate .php file and callin the image with a <IMG tag, but after messing with magic quotes/addslashes (to handle when I submitted non-text characters) decided to go the 'single script route' (which worked for what I wanted to do).

The script above is a stripped down example of what I'm working with, and it does what I'm trying to do with one exception: When it displays the generated picture, if I try to save the picture by right-clicking and selecting "save as", it isn't saving the picture. It only saves the php-output of the form.

Why is it doing that / what's the best way to get just that functionality?